Contribution of Particulate Phosphorus to Runoff Phosphorus Bioavailability

Runoff P associated with eroded soil is partly solubilized in receiving waters and contributes to eutrophication, but the significance of particulate phosphorus (PP) in the eutrophying P load is debatable. We assessed losses of bioavailable P fractions in field runoff from fine-textured soils (Cryaquepts). Surface runoff at four sites and drainflow at two of them was sampled. In addition to dissolved molybdate-reactive phosphorus (DRP) losses, two estimates of bioavailable PP losses were made: (i) desorbable PP, assessed by anion exchange resin-extraction (AER-PP) and (ii) redox-sensitive PP, assessed by extraction with bicarbonate and dithionite (BD-PP). Annual losses of BD-PP and AER-PP were derived from the relationships (R^2 = 0.77–0.96) between PP and these P forms. Losses of BD-PP in surface runoff (94–1340 g ha^-1) were typically threefold to fivefold those of DRP (29–510 kg ha^-1) or AER-PP (13–270 g ha^-1). Where monitored, drainflow P losses were substantial, at one of the sites even far greater than those via the surface pathway. Typical runoff DRP concentration at the site with the highest Olsen-P status (69–82 mg kg^-1) was about 10-fold that at the site with the lowest Olsen P (31–45 mg kg^-1), whereas the difference in AER-PP per mass unit of sediment was only threefold, and that of BD-PP 2.5-fold. Bioavailable P losses were greatly influenced by PP runoff, especially so on soils with a moderate P status that produced runoff with a relatively low DRP concentration.